Sputtering is an electric discharge process used to synthesize materials while imparting certain characteristics thereto. For example, sputtering may be used to synthesize glass while, concurrently, providing the glass with characteristics such as emissivity, reflectivity, transmittance, chemical resistance or color.
During the sputtering process, gas ions bombard a target having a face formed of a desired material that is to be deposited or coated as a thin film or layer on a substrate. The ion bombardment of the target causes atoms or molecules of the target material to be ejected (or "sputtered") onto the substrate to synthesize a material. The target may be specifically selected in view of the material to be synthesized and the characteristics to be provided to the synthesized material.
Silicon-aluminum alloys are often used as targets in sputtering processes to synthesize glass because the addition of the aluminum advantageously increases the electrical conductivity of the silicon. But, current methods for preparing such silicon-aluminum targets frequently result in targets that have several shortcomings which translate into serious drawbacks in glass synthesized using those targets.
Conventional silicon-aluminum targets are prepared by vacuum casting methods in which the targets are solidified from a 100% liquid state. This preparation technique results in target materials having certain undesirable characteristics such as grain size control difficulty, heterogeneity, or segregation. In turn, glass sputter-coated using these targets tends to have defects visible in the coating, thus rendering the glass useless.
A characteristic that is particularly undesirable, yet also difficult to avoid during vacuum-casting preparation of a silicon-aluminum target, is residual porosity at the center of the target. A sputtering process that is carried out with a silicon-aluminum target having a residual porosity of as little as 0.5 millimeters in diameter at the center of the target could cause electrical arcing during the sputtering process. In turn, glass sputter-coated using a silicon-aluminum target with residual porosity, has a reduced yield, and exhibits visible defects in the coating.
It would, therefore, be desirable to provide a silicon-aluminum sputtering target and a method for fabricating such a target, which provide certain enhanced characteristics to glass sputter-coated using the target.